1. Field of the Invention:
This invention relates to a method for the production of a water absorbent resin. More particularly, it relates to a method for the production of a water absorbent resin which is excellent in absorption properties.
2. Description of the Prior Art:
As water absorbent resins obtainable by the polymerization of a monomer, cross-linked polyacrylates, saponified acrylic ester-vinyl acetate copolymers, modified cross-linked polyvinyl alcohols, partially neutralized cross-linked polyacrylates, cross-linked isobutylene-maleic anhydride copolymers, and starch-acrylic acid graft copolymer have been known in the art. They are extensively utilized as sanitary absorbents such as sanitary napkins and disposable diapers and as water-retaining agents, dehydrating agents, etc. in the field of agriculture and horticulture and in the field of civil engineering, for example.
Among various methods heretofore available for the production of such water absorbent resins, there resorting to the principle of reverse phase suspension polymerization are disclosed in Japanese Patent Laid-Opens SHO Nos. 56(1981)-161,408, SHO 57(1982)-94,011, SHO 57(1982)-158,209, and SHO 57(1982)-198,714, for example, and those resorting to the principle of aqueous solution polymerization are disclosed in Japanese Patent Laid-Open SHO No. 57(1982)-34,101, Japanese Patent Publication SHO No. 48(1973)-42,466, Japanese Patent Laid-Open SHO No. 58(1983)-49,714, Japanese Patent Publication SHO No. 59(1984)-37,003, U.S. Pat. Nos. 4,286,082, and 4,625,001, for example.
The method of reverse phase suspension polymerization necessitates use of an organic solvent and, therefore, entails disadvantages that the organic solvent not only impair the working environment but also threaten explosion due to flashing. Measures must be taken against these disadvantages. The measures render the operation of reverse phase suspension polymerization expensive together with the expense of the organic solvent itself and the expense for the removal of spent solvent. Further, it has a drawback that since the restriction imposed on solid concentration, the productivity of the operation is degraded. Since this organic solvent tends to remain, though in a minute amount, in the product of reverse phase suspension polymerization, an attempt at ensuring perfect removal of the residual organic solvent from the product renders the method all the more expensive. Moreover, since the absorbent resin produced by the method of reverse phase suspension polymerization is in the form of beads of an unduly small diameter, the resin beads used in disposable diaper, for example, have the possibility of falling off the fibrous absorbent core component such as pulp. This resin suffers inconvenience in handling from the fact that it is in the form of minute beads. Further, the method of reverse phase suspension polymerization entails the disadvantage that when the cross-linking agent of insufficient hydrophilicity is used, this cross-linking agent tends to dissolve out in the hydrophobic organic solvent. Though it is possible to increase the solubility of the cross-linking agent in an aqueous monomer solution by the decrease of the neutralization degree of the monomer solution, when this case, it has a problem that the acid group-containing monomer tend to dissolve out in the hydrophobic organic solvent and induce flocculation.
The method of aqueous solution polymerization suffers from none of the drawbacks mentioned above. Particularly the methods disclosed in Japanese Patent Laid-Open SHO No. 57(1982)-34,101 and U.S. Pat. No. 4,625,001 are those for producing a cross-linked polymer by a procedure which comprises effecting radical aqueous solution polymerization of the aqueous solution of a monomer capable of forming a cross-linked structure in the molecular unit thereof on exposure to aqueous solution polymerization and consequently converting itself into a hydrated gel polymer in combination, with a polymerization initiator in a reaction vessel provided with stirring blades by finely dividing the hydrated gel polymer produced in consequence of the progress of the polymerization with the shearing force of the stirring blades generated by the rotation of the stirring shaft. According to these methods, they enjoy extremely high operational efficiency and also have an advantage that finely divided hydrated gel polymer possessing a cross-linked structure in the molecular unit thereof can be produced with ease. Even in these methods of such characteristic features, it has possibility that the production of an absorbent resin exhibiting a high capacity for absorption and containing water solubles in a small amount is difficult.
It is widely known to persons skilled in the art that the ratio of absorption of a given resin is increased by lowering the density of cross-linking. It is also well known that when a water absorbent resin is produced by a procedure capable of lowering the cross-linking density, the produced water absorbent resin suffers from an increase in the content of water solubles. The water solubles are of such nature that when the water absorbent resin forms a hydrogel structure on contact with various liquids such as water, urine, and body fluid, they are exuded through the hydrogel structure. The water solubles which are thus extracted by the liquids subjected to absorption not only degrades the water absorbent resin's ratio of absorption but also accelerates the deterioration of the water absorbent resin. Moreover, the viscid consistency of the water solubles gives birth to a disadvantage that it excites an unpleasant sensation in the human skin and smears the liquid subjected to absorption.
In the circumstances, the desirability of perfecting a method for the production of a water absorbent resin enjoying a high ratio of absorption and containing water solubles only in a small amount has been finding growing approval. U.S. Pat. No. 4,654,039 discloses a method for producing a water absorbent resin enjoying a high ratio of absorption and containing water insolubles only in a small amount by subjecting a substantially free acid type monomer to aqueous solution polymerization. It has been demonstrated, however, that when an unneutralized monomer is polymerized by the use of a conventional radical polymerization initiator, the amount of residual monomer increases in the produced polymer. Naturally, the increase in the amount of residual monomer is nothing to be desired from the standpoint of safety.
An object of this invention, therefore, is to provide a novel method for the production of a water absorbent resin.
Another object of this invention is to provide a method for the production of a water absorbent resin excelling in absorption properties and containing an residual monomer only in a small amount.